Low power discharge lamp with high efficacy

ABSTRACT

In order to achieve a discharge lamp suited to operate under reduced nominal power of e.g. 20-30 W, a lamp is proposed with two electrodes ( 24 ) arranged at a distance in a discharge vessel ( 20, 120 ) for generating an arc discharge. The discharge vessel ( 20,120 ) has a filling with a substantially free of mercury and comprises a metal halide and a rare gas. The lamp ( 10, 110 ) further comprises an outer bulb ( 18 ) arranged around the discharge vessel at a distance (d 2 ). The outer bulb ( 18 ) is sealed and has a gas filling of a thermal conductivity (λ). The inner diameter (d 1 ) of the discharge vessel is preferably in a range from 2-2.7 mm. The wall thickness (w 1 ) is in a range from 1.4-2 mm. A heat transition coefficient (λ/d 2 ) is calculated as thermal conductivity (λ) at 800° C. of the outer bulb filling divided by the distance (d 2 ). The so-defined heat  10  transition coefficient is below 150 W/(m 2 K).

FIELD OF THE INVENTION

The invention relates to a discharge lamp. More specifically, theinvention relates to a high intensity discharge lamp with a dischargevessel and an outer bulb arranged around the discharge vessel.

BACKGROUND OF THE INVENTION

Discharge lamps, specifically HID (high-intensity discharge) lamps areused for a large area of applications where energy efficiency and highlight intensity are required. Especially in the automotive field, HIDlamps are used as vehicle headlamps.

A discharge lamp comprises two electrodes arranged at a distance withina discharge vessel. An arc discharge is generated between theelectrodes. Different types of fillings within the discharge vessel areknown, distinguishing mercury vapor, metal halide and other types oflamps.

Commercially available lamps for use in a vehicle head-light have anouter bulb which is arranged around the discharge vessel at a distancetherefrom. A known type of such a lamp is designed for a nominal powerof 35 W and achieves a high efficacy of 80-90 lm/W. After starting sucha lamp, a run-up current of, for example, 2.7-3.2 A is necessary, and arun-up power of 75-80 W is used. Thus, the complete HID systemcomprising lamp, ballast and igniter must be able to operate as thesevalues.

Especially for the automotive field, it would be desirable to have adischarge lamp with lower nominal power, e.g. in the range of 20-30 W,and correspondingly lower demands on the complete HID system. If,however, known lamp designs are simply used at lower power, the lampefficacy will be dramatically reduced.

US-A-2005/0248278 shows an example of an automotive head lightingdischarge lamp with a power of 30 W. The lamp has a ceramic dischargevessel comprising the electrodes, which is surrounded by an outer bulb.The distance between the electrode tips is 5 mm. The discharge vesselhas cylindrical shape with an internal diameter of 1.2 mm. The wallthickness of the discharge vessel is 0.4 mm. The discharge vesselcomprises a filling which is free from mercury and comprises NaPrI andZnI₂ as well as Xe with a filling pressure of 16 bar. The outer bulb ismade of quartz glass and is arranged at a distance of 0.5 mm to thedischarge vessel. The outer bulb is filled with N₂ with a fillingpressure of 1.5 bar at room temperature.

It is an object of the invention to provide a relatively low power HIDlamp with high lamp efficacy.

This object is achieved by a high intensity discharge lamp according toclaim 1. Dependent claims refer to preferred embodiments of theinvention.

SUMMARY OF THE INVENTION

The inventors have recognized that in order to maintain high efficacythermal design of the lamp needs to be adapted to the lower power. The“coldest spot”-temperature needs to be maintained at a high level toachieve good lamp efficacy. However, thermal load on a “hot spot” needsto be constrained in order to achieve good durability. This has led theinventors to propose a lamp with a relatively small discharge vessel,leading to reduced heat radiation, while still maintaining asufficiently thick wall of the discharge vessel to not only withstandhigh internal pressure, but specifically to allow heat conduction fromthe hot upper side (“hot spot”) to the colder lower side.

According to the invention, a specific geometry is provided in view ofthe thermal design of the lamp. The discharge vessel is maintained witha substantial wall thickness of 1.4-2 mm, and preferably also arelatively small inner diameter from 2-2.7 mm.

An outer bulb is arranged around the discharge vessel. The outer bulb issealed and has a gas filling with a thermal conductivity λ. The thermalconductivity λ of the outer bulb filling is taken at 800° C.

The geometry of the outer bulb (here specifically: the distance d₂between the discharge vessel and the outer bulb) and the gas filling arechosen to achieve a certain, limited heat flow from the discharge vesselto the outside. The thermal conductivity λ of the gas filling and thedistance d₂ are chosen to obtain a desired heat transition coefficientλ/d₂ calculated as the thermal conductivity λ divided by the distanced₂. According to the invention, this coefficient is below 150 W/(m²K).For the purposes of measurement, here, the distance d₂ is measured incross-section of the lamp taken at a central position between theelectrodes.

The outer bulb therefore plays an important part in the thermal designof the lamp. While on one hand thermal radiation is limited by thelimited size of the discharge vessel, heat conduction in radialdirection of the lamp is further limited by the geometry and filling ofthe outer bulb. As will be explained in relation to the preferredembodiment, the amount of heat transported per time unit between thedischarge vessel and the outer bulb, both at their constant operatingtemperature, is roughly proportional to the defined heat transitioncoefficient. Thus, by choosing the heat transition coefficient to bebelow 150 W/(m²K), cooling is limited, such that sufficient high coldestspot temperatures, and thus high efficacy are maintained. To achieve adesired, high enough coldest spot temperature the heat transitioncoefficient is preferably equal to or less than 130 W/(m²K), mostpreferably even lower <100 W/(m²K). It is further preferred for the heattransition coefficient to be at least 10 W/(m²K), further preferred atleast 15 W/(m²K).

A lamp according to the invention is especially suited for a nominalpower of 20-30 W. The filling of the discharge vessel is preferably freeof mercury and may comprise one or more metal halides and a rare gas.Preferably, the filling of the discharge vessel comprises one or more ofthe following: NaI, ScI₃, ZnI₂.

Preferred embodiments of the invention relate to the outer bulb. Theouter bulb is preferably made out of quartz glass and may be of anygeometry, e.g. cylindrical, generally elliptical or other. It ispreferred for the outer bulb to have an outer diameter of at most 10 mm.The outer bulb is sealed and has a gas filling at a pressure of 10 mbarto 1 bar, preferably below 1 bar, most preferably 50 mbar to 300 mbar.The gas filling may essentially consist (i.e. comprise more than 50%,preferably more than 90%) of one or more of the following: Xe, Ar, N₂,O₂. The distance d₂ between the outer bulb and the discharge vessel ispreferably 0.1-1.4 mm, most preferably 0.3-0.8 mm. As will beappreciated by the skilled person, the filling gas, pressure anddistance d₂ may only be chosen dependent on one another to achieve thedesired heat transition coefficient.

Other preferred embodiments of the invention relate to the dischargevessel. Preferably, the discharge vessel is made from quartz glass. Thedistance between the electrodes is preferably 2.5-5.5 mm. Mostpreferably, the optical distance (i.e. the distance as viewed from theoutside, taking into account magnification of the discharge vessel wallacting as a lens) is 4.2±0.6 mm. The discharge vessel has a shape suchthat in a cross-section taken at the central position between theelectrodes the wall of the discharge vessel is at least substantiallycircular.

In a preferred embodiment, the discharge vessel, when viewed inlongitudinal section, has at least substantially elliptical outer shapeand may have either elliptical or cylindrical inner shape. In this case,it is preferred for the wall thickness w₁ to be in the range from1.55-1.85 mm.

According to an alternative embodiment, the discharge vessel, whenviewed in longitudinal section, has elliptical or cylindrical innershape and concave outer shape, i.e. starting from the central positionbetween the electrodes the outer diameter of the discharge vesselincreases towards both sides. In this case, it is preferred for the wallthickness w₁ to be in the range from 1.4-2 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following description ofpreferred embodiments, in which:

FIG. 1 shows a side view of a lamp according to a first embodiment ofthe invention;

FIG. 2 shows an enlarged view of the central portion of the lamp shownin FIG. 1;

FIG. 2a shows a cross-sectional view along the line A in FIG. 2;

FIG. 3 shows a side view of a lamp according to a second embodiment ofthe invention;

FIG. 4 shows a side view of a lamp according to a third embodiment ofthe invention;

FIG. 5 shows an enlarged view of the central portion of the lamp shownin FIG. 4;

FIG. 5a shows a cross-sectional view along the line A in FIG. 5,

FIG. 6 shows a side view of a lamp according to a fourth embodiment ofthe invention,

FIG. 7 shows a graph representing a heat transition coefficient λ/d₂ fordifferent fillings and distances d₂, and

FIG. 8 shows a graph representing measured values of lumen output overtime (run-up) for a lamp according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

All embodiments shown are intended to be used as automotive lamps forvehicle head lights, conforming to ECE R99 and ECE R98. This,specifically, is not intended to exclude lamps for non-automotive use,or lamps according to other regulations. Since such automotive HID lampsare known per se, the following description of the preferred embodimentswill primarily focus on the special features of the invention.

FIG. 1 shows a side view of a first embodiment 10 of a discharge lamp.The lamp comprises a socket 12 with two electrical contacts 14 which areinternally connected to a burner 16.

The burner 16 is comprised of an outer bulb 18 of quartz glasssurrounding a discharge vessel 20. The discharge vessel 20 is also madeof quartz glass and defines an inner discharge space 22 with projectingelectrodes 24. The glass material from the discharge vessel furtherextends in longitudinal direction of the lamp 10 to seal the electricalconnections to the electrodes 24 which comprise a flat molybdenum foil26.

The outer bulb 18 is arranged around the discharge vessel 20 at adistance, thus defining an outer bulb space 28. The outer bulb space 28is sealed.

As shown in greater detail in FIG. 2, the discharge vessel 20 has anouter wall 30 arranged around the discharge space 22. The dischargespace 22 is of ellipsoid shape. Also, the outer shape of the wall 30 isellipsoid.

The discharge vessel 20 is characterized by the electrode distance d,the inner diameter d₁ of the discharge vessel 20, the wall thickness w₁of the discharge vessel, the distance d₂ between the discharge vessel 20and the outer bulb 18 and the wall thickness w₂ of the outer bulb 18.Here, the values d₁, w₁, d₂, w₂ are measured in a central perpendicularplane of the discharge vessel 20, as shown in FIG. 2a.

The lamp 10 is operated, as conventional for a discharge lamp, byigniting an arc discharge between the electrodes 24. Light generation isinfluenced by the filling comprised within the discharge space 22, whichis free of mercury and includes metal halides as well as a rare gas.

In the following examples, the filling of the discharge space 22comprises about 17 bar cold xenon pressure and as metal halides 36 wt %NaI, 24 wt % ScI₃ and 40 wt % ZnI₂.

In the following, different embodiments of a lamp will be discussed,which are each intended to be used at different (steady-state) levels ofoperating power. The operating power of the embodiments is within theinterval of 25-30 W. For each embodiment, a specific design is chosenwith regard to thermal characteristics of the lamp in order to achievehigh lamp efficacy.

Regarding the thermal behavior of a discharge lamp 10 as shown, itshould be kept in mind that automotive lamps are intended to be operatedhorizontally. The arc discharge between the electrode 24 will then leadto a hot spot at the wall 30 of the discharge vessel 20 above the arc.Likewise, opposed portions of the wall 30 surrounding the dischargespace 22 will remain at comparatively low temperatures (coldest spot).

In order to achieve good efficacy and, as will become apparent later,also achieve favorable run-up behavior, the geometric design of the lamp10 is chosen according to thermal considerations. The “coldest spot”temperature should be kept high to achieve high efficacy. The thicknessof the wall 30 should be small enough to allow a quick run-up withlimited run-up current, but should not be too small in order to stillachieve good heat conduction from the “hot spot” in order to reducethermal load. The inner diameter d₁ should not be too small in order toreduce excessive thermal load at the “hot spot”.

In order to reduce heat transport from the discharge vessel 20 to theoutside, and to maintain high temperatures necessary for good efficacy,it is thus preferable to use the outer bulb 18 instead of a significantreduction of the thickness w₁ of the wall 30. In contrast to a simpledownscaling of the discharge vessel 20 (reduced inner diameter, reducedwall thickness, reduced outer diameter), this has proven to also serveto maintain a good lamp lifetime.

In order to limit cooling from the outside, the outer bulb 18 is sealedand filled with a filling gas of reduced heat conductivity. EspeciallyArgon and Xenon are preferred here, but O₂ or N₂ could be used as well.The outer bulb filling is provided at reduced pressure (measured in thecold state of the lamp at 20° C.). As will be further explained below,the choice of a suitable filling gas has to be made in connection withthe geometric arrangement in order to achieve the desired heatconduction from discharge vessel 20 to outer bulb 18 via a suitable heattransition coefficient λ/d₂.

In the following table, measurement results of lamp efficacy are shownfor a lamp as shown in FIG. 1-2a with an inner diameter d₁=2.2 mm, awall thickness w₁ of 1.65 mm (thus an outer diameter of the dischargevessel of 5.5 mm) and a steady-state operating power of 25 W fordifferent outer bulb fillings:

Efficacy Coldest spot Outer bulb filling S-type temperature (outside)Air (1 bar) 67 lm/W 810° C. Ar (100 mbar) 79 lm/W 840° C. Xe (100 mbar)86 lm/W 900° C.

It is thus clearly visible how the reduced heat conduction to theoutside leads to a higher coldest spot temperature, and to a higher lampefficacy.

The heat conduction to the outside may be roughly characterized by aheat transition coefficient λ/d₂, which is calculated as the thermalconductivity λ of the outer bulb filling divided by the distance d₂between the discharge vessel 20 and the outer bulb 18.

Due to the relatively small distance between the discharge vessel 20 andouter bulb 18, heat conduction between the two is essentially diffusiveand will therefore be calculated as {dot over (q)}=−λ grad Θ, where {dotover (q)} is the heat flux density, i.e. the amount of heat transportedper time between discharge vessel and outer bulb. λ is the thermalconductivity and grad Θ, is the temperature gradient, which here mayroughly be calculated as the temperature difference between dischargevessel and outer bulb, divided by the distance:

${{grad}\;\vartheta} = {\frac{T_{dischargeVessel} - T_{outerBulb}}{d_{2}}.}$Thus, cooling is proportional to

$\frac{\lambda}{d_{2}}.$

FIG. 7 shows the dependence of the heat transition coefficient λ/d₂ onthe distance d₂ for different outer bulb fillings. It is clearly visiblehow Argon, and especially Xenon (provided here at a reduced pressure of200 mbar) have significantly lower heat conductivity than air, and thatthe heat transition coefficient λ/d₂ is further reduced with increasingdistance d₂. The heat transition coefficient was found to differ morestrongly with the gas composition, and less with the pressure, if it isin the range from about 10 mbar to about 1 bar.

The following examples of lamps with a rated power of 25-30 W areproposed:

Example 1

25 W lamp discharge vessel: ellipsoid inner and outer shape electrodedistance d = 4.2 mm optical inner diameter d₁ = 2.2 mm wall thickness w₁= 1.65 mm outer diameter = 5.5 mm outer bulb distance d₂ = 0.6 mm outerbulb filling = Xe 100 mbar ( λ = 0.014 W/(m*K) at 800° C.) heattransition coefficient λ/d₂ = 23.3 W/(m²K) at 800° C. outer bulb wallthickness w₂ = 1 mm

Example 2

30 W lamp discharge vessel: ellipsoid inner and outer shape electrodedistance d = 4.2 mm optical inner diameter d₁ = 2.3 mm wall thickness w₁= 1.75 mm outer diameter = 5.8 mm outer bulb distance d₂ = 0.45 mm outerbulb filling = Xe 100 mbar ( λ = 0.014 W/(m*K) at 800° C.) heattransition coefficient λ/d₂ = 31.1 W/(m²K) at 800° C. outer bulb wallthickness w₂ = 1 mm

FIG. 3 shows a second embodiment of the invention. A lamp 110 accordingto the second embodiment comprises a discharge vessel 120 of differentinternal shape. The remaining parts of the lamp correspond to the lamp10 according to the first embodiment. Like elements will be designatedby like reference numerals, and will not be further described in detail.

The discharge vessel 120 of the lamp 110 has external ellipsoid shape,identical to the discharge vessel 20 according to the first embodiment.However, the internal discharge space 22 is cylindrical. Both the lengthand diameter of the inner discharge space 22 however are as in the abovefirst embodiment. It should be noted that the term “cylindrical” usedhere refers to the central, largest part of the discharge space 22 anddoes not exclude—as shown—conical end portions.

The wall 130 surrounding the discharge space 22 is consequently ofvarying thickness, with the thickness being greatest at a positioncorresponding to the center between the electrodes 24, and decreasingtowards both sides.

In the following, a third embodiment of the invention will be describedwith reference to FIGS. 3-4a. A lamp 110 according to the secondembodiment again in large parts corresponds to the lamp 10 according tothe above first and second embodiments. Like elements will be designatedby like reference numerals and will not be further described in detail.

The lamp 210 differs from the lamp 10 by the concave outer shape of thedischarge vessel 120. The inner discharge space 22 remains roughlyellipsoidal as in the first embodiment. However, the wall 230surrounding the discharge space 22 has a varying wall thickness suchthat its outer shape is concave.

Again, geometrical parameters d₁, w₁, d₂, w₂ are measured in a centralplane of the discharge vessel 220.

FIG. 6 shows a fourth embodiment of the invention, which in large partscorresponds to the third embodiment according to FIG. 4-5a. Again, likeelements are designated by like reference numerals and will not befurther described in detail.

According to the fourth embodiment of the invention, a lamp 310 has adischarge vessel 320 with a concave outer shape, but an inner dischargespace 22 of cylindrical shape.

Both in the third and forth embodiment, the thickness of the wall 230,330 surrounding the discharge space 22 varies such that it is minimal ina position corresponding to the center between the electrodes 24 andincreases towards both sides. This leads to a lens effect, such that theelectrode distance d will appear to the outside smaller than it actuallyis. Thus, to achieve the desired optical electrode distanced of 4.2 mm,the real electrode distance may be, e.g. 4.8 mm in the third and in theforth embodiment. The possibility to thus increase the real electrodedistance d but maintain the optical distance gives to the lamp designera further degree of freedom. Since the operating voltage increases withthe electrode distance, it is possible to obtain a higher voltage.

This may be used to provide a lamp which is compatible with ECE R99geometrically (optical distance 4.2 mm), but—as amercury-free-lamp—fulfills the electric requirements of a D2 lamp(voltage more than 68 V).

On the other hand, for the first and second embodiment (elliptical outershape), it is also possible to provide a larger electrode distance toobtain a lamp, which is not according to ECE R99, but may be operatedwith higher voltage.

The following examples of lamps according to the third embodiment in arange of 25-30 W are proposed:

Example 3

25 W lamp discharge vessel: concave outer shape, ellipitical inner shapeelectrode distance d = 4.2 mm optical inner diameter d₁ = 2.2 mm wallthickness w₁ = 1.5 mm outer diameter = 5.2 mm outer bulb distance d₂ =0.75 mm outer bulb filling = Ar 100 mbar ( λ = 0.045 W/(m*K) at 800° C.)heat transition coefficient λ/d₂ = 60 W/(m²K) at 800° C. outer bulb wallthickness w₂ = 1 mm

Example 4

28 W lamp discharge vessel: concave outer shape, elliptical inner shapeelectrode distance d = 4.2 mm optical inner diameter d₁ = 2.2 mm wallthickness w₁ = 1.7 mm outer diameter = 5.6 mm outer bulb distance d₂ =0.55 mm outer bulb filling = 50% Ar/50% Xe 100 mbar ( λ = 0.045 W/(m*K)at 800° C.) heat transition coefficient λ/d₂ = 45.5 W/(m²K) at 800° C.outer bulb wall thickness w₂ = 1 mm

Example 5

30 W lamp discharge vessel: concave outer shape, elliptical inner shapeelectrode distance d = 4.2 mm optical inner diameter d₁ = 2.2 mm wallthickness w₁ = 1.9 mm outer diameter = 6.0 mm outer bulb distance d₂ =0.35 mm outer bulb filling = 50% Ar/50% Xe 100 mbar ( λ = 0.025 W/(m*K)at 800° C.) heat transition coefficient λ/d₂ = 71.4 W/(m²K) at 800° C.outer bulb wall thickness w₂ = 1 mm

In the above examples, only discharge vessels of elliptical inner shapewere used. However, the same measurements may be used for cylindricalinner shape.

FIG. 8 shows measurement results of run-up tests, where a 25 W lampaccording to the above example 1 was compared to a reference lamp (35 Wlamp). The lumen output was measured and is shown in FIG. 8 over thetime since ignition of the lamp. As is known for starting the lamps, ina first phase, the current is limited to a maximum value, and in asecond phase, the power is controlled.

As shown in FIG. 8, the reference lamp reaches about 50% of the totallumen output after 4 seconds. But this requires a maximum run-up currentof 3.2 A, resp. a maximum power of around 75 W. The 25 W lamp accordingto example 1 was first driven with a current limitation in the firstphase of 1.1 A. Here, the results (less then 30% after 4 seconds) werenot satisfactory. However, with a run-up current limitation of 1.5 A(maximum power about 50 W), the lamp according to example 1 shows aquite comparable behavior to the reference, whereas the run-up currentis less then half and the maximum run-up power is reduced by about 30%.

The remaining examples where found to also show satisfactory behaviorwith a run-up current significantly lower then necessary for thereference lamp. This is due to the fact that the smaller dischargevessel is heated up quickly by the arc discharge.

As lifetime tests have shown, the lifetime performance within the first1500 hours of operation for lamps according to the above embodimentscorresponds to the reference (a 35 W lamp).

Thus, it has been shown that the above embodiments provide lamps withgood lifetime, good efficacy and good run-up behavior, which allcorrespond to the reference lamps, but at lower required run-up currentand lower steady-state power.

The invention has been illustrated and described in detail in thedrawings and foregoing description. Such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments.

In the claims, the word “comprising” does not exclude other elements,and the indefinite article “a” or “an” does not exclude a plurality. Themere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage. Any reference signs in the claims shouldnot be construed as limiting the scope.

The invention claimed is:
 1. A high intensity discharge lamp,comprising: a discharge vessel defining a discharge space beingessentially free of mercury and containing at least a metal halide and arare gas; two electrodes disposed within the discharge vessel forgenerating an arc discharge a wall forming the discharge space disposedbetween the electrodes, the wall having substantially circularcross-section with an inner diameter (d₁) and a wall thickness (w₁), andan outer bulb surrounding the discharge vessel and disposed at adistance (d₂) from a central position of an outer surface of thedischarge vessel between the electrodes, the outer bulb being sealed andcontaining a gas filling having a predetermined thermal conductivity (λ)at 800° C., wherein the wall thickness (w₁) ranges from about 1.4 mm toabout 2 mm, the distance (d2) ranges from about 0.3 mm to about 0.8 mm,and a heat transition coefficient (λ/d₂) calculated as said thermalconductivity (λ) divided by said distance (d₂) ranges from about 10W/(m²K) to about 100 W/(m²K).
 2. Lamp according to claim 1, wherein saidinner diameter (d₁) ranges from about 2 mm to about 2.7 mm.
 3. Lampaccording to claim 1, wherein the gas filing consists essentially of Xe,Ar, N₂, or O₂.
 4. Lamp according to claim 1, wherein the gas filling hasa pressure of 10 mbar to 1 bar.
 5. Lamp according to claim 1, whereinthe gas filling has a lower thermal conductivity at 800° C. than air. 6.Lamp according to claim 1, wherein the wall thickness (w₁) ranges fromabout 1.55 mm to about 1.85 mm.
 7. Lamp according to claim 1, whereinsaid lamp has a nominal power 20-30 W.
 8. Lamp according to claim 1,wherein said discharge vessel is made of quartz glass.
 9. Lamp accordingto claim 1, wherein said outer bulb is made of quartz glass.
 10. A highintensity discharge lamp, comprising: a discharge vessel defining adischarge space being essentially free of mercury and containing atleast a metal halide and a rare gas; two electrodes disposed within thedischarge vessel for generating an arc discharge a wall forming thedischarge space disposed between the electrodes, the wall havingsubstantially circular cross-section with an inner diameter (d1) and awall thickness (w1), and an outer bulb surrounding the discharge vesseland disposed at a distance (d2) from a central position of an outersurface of the discharge vessel between the electrodes, the outer bulbbeing sealed and containing a gas filling having a predetermined thermalconductivity (λ) at 800° C., wherein the wall thickness (w1) ranges fromabout 1.4 mm to about 2 mm, the distance (d2) ranges from about 0.1 mmto about 1.4 mm, and a heat transition coefficient (λ/d2) calculated assaid thermal conductivity (λ) divided by said distance (d2) ranges fromabout 10 W/(m2K) to about 100 W/(m2K).
 11. Lamp according to claim 10,wherein said inner diameter (d1) ranges from about 2 mm to about 2.7 mm.12. Lamp according to claim 10, wherein the gas filing consistsessentially of Xe, Ar, N2, or O2.
 13. Lamp according to claim 10,wherein the gas filling has a pressure of 10 mbar to 1 bar.
 14. Lampaccording to claim 10, wherein the gas filling has a lower thermalconductivity at 800° C. than air.
 15. Lamp according to claim 10,wherein the wall thickness (w1) ranges from about 1.55 mm to about 1.85mm.
 16. Lamp according to claim 10, wherein said lamp has a nominalpower from about 20 W to about 30 W.
 17. Lamp according to claim 10,wherein said discharge vessel is made of quartz glass.
 18. Lampaccording to claim 10, wherein said outer bulb is made of quartz glass.19. A high intensity discharge lamp, comprising: a discharge vesseldefining a discharge space being essentially free of mercury andcontaining at least a metal halide and a rare gas; two electrodesdisposed within the discharge vessel for generating an arc discharge awall forming the discharge space disposed between the electrodes, thewall having substantially circular cross-section with an inner diameter(d1) and a wall thickness (w1), and an outer bulb surrounding thedischarge vessel and disposed at a distance (d2) from a central positionof an outer surface of the discharge vessel between the electrodes, theouter bulb being sealed and containing a gas filling having apredetermined thermal conductivity (λ) at 800° C., wherein the wallthickness (w1) ranges from about 1.4 mm to about 2 mm, the distance (d2)is selected so that a heat transition coefficient (λ/d2) calculated assaid thermal conductivity (λ) divided by said distance (d2) ranges fromabout 10 W/(m2K) to about 100 W/(m2K).
 20. Lamp according to claim 19,wherein said inner diameter (d1) ranges from about 2 mm to about 2.7 mm.21. Lamp according to claim 19, wherein the gas filing consistsessentially of Xe, Ar, N2, or O2.
 22. Lamp according to claim 19,wherein the gas filling has a pressure of 10 mbar to 1 bar.
 23. Lampaccording to claim 19, wherein the gas filling has a lower thermalconductivity at 800° C. than air.
 24. Lamp according to claim 19,wherein the wall thickness (w1) ranges from about 1.55 mm to about 1.85mm.
 25. Lamp according to claim 19, wherein said lamp has a nominalpower from about 20 W to about 30 W.
 26. Lamp according to claim 19,wherein said discharge vessel is made of quartz glass.
 27. Lampaccording to claim 19, wherein said outer bulb is made of quartz glass.28. A high intensity discharge lamp, comprising: a discharge vesseldefining a discharge space being essentially free of mercury andcontaining at least a metal halide and a rare gas; two electrodesdisposed within the discharge vessel for generating an arc discharge awall forming the discharge space disposed between the electrodes, thewall having substantially circular cross-section with an inner diameter(d1) and a wall thickness (w1), and an outer bulb surrounding thedischarge vessel and disposed at a distance (d2) from a central positionof an outer surface of the discharge vessel between the electrodes, theouter bulb being sealed and containing a gas filling having apredetermined thermal conductivity (λ) at 800° C., wherein the wallthickness (w1) and the distance (d2) are selected so that a heattransition coefficient (λ/d2) calculated as said thermal conductivity(λ) divided by said distance (d2) ranges from about 10 W/(m2K) to about100 W/(m2K).
 29. Lamp according to claim 28, wherein said inner diameter(d1) ranges from about 2 mm to about 2.7 mm.
 30. Lamp according to claim28, wherein the gas filing consists essentially of Xe, Ar, N2, or O2.31. Lamp according to claim 28, wherein the gas filling has a pressureof 10 mbar to 1 bar.
 32. Lamp according to claim 28, wherein the gasfilling has a lower thermal conductivity at 800° C. than air.
 33. Lampaccording to claim 28, wherein the wall thickness (w1) ranges from about1.55 mm to about 1.85 mm.
 34. Lamp according to claim 28, wherein thedistance (d2) ranges from about 0.1 mm to about 1.4 mm.
 35. Lampaccording to claim 28, wherein said lamp has a nominal power from about20 W to about 30 W.
 36. Lamp according to claim 28, wherein saiddischarge vessel is made of quartz glass.
 37. Lamp according to claim28, wherein said outer bulb is made of quartz glass.